Vehicles may be configured with downhill speed control systems that allow a vehicle speed to be maintained when traveling downhill by limiting vehicle acceleration on the downhill grade. For example, in hybrid electric vehicles (HEVs) that can use an engine or a battery-powered motor to propel the vehicle, various approaches involving wheel braking and regenerative braking can be used to control the vehicle speed.
One example approach is shown by Jamzadeh in U.S. Pat. No. 7,410,447. Therein, a speed control system is adapted to absorb the grade-induced energy so that the vehicle does not accelerate and an established vehicle speed can be maintained. Vehicle speed control is automatically triggered responsive to a predefined amount of throttle release. Further, the amount of energy absorbed is based on the difference between the desired speed and the actual vehicle speed. In one example, the grade-induced energy may be absorbed by an electric motor/regenerator of the hybrid electric vehicle and stored in the attached battery or accumulator. Alternatively, the grade-induced energy may be absorbed by internal clutch elements or wheel brakes.
However, the inventors herein have identified a potential issue with such an approach. The operating life and performance characteristics of a system battery of the hybrid electric vehicle may be degraded. Specifically, during regenerative braking, irrespective of the grade of the vehicle, the battery may be charged as fast as possible using faster and deeper rates of charging and higher current levels. Rapid charging or discharging of the battery leads to battery heating which may adversely affect the operating life and storage capacity of the battery, as well as battery performance over the rest of the trip. Reduced battery performance may also affect the performance and fuel efficiency of the electric vehicle.
In one example, the above issue may be at least partly addressed by a method for a vehicle including an engine and a battery-powered motor. In one embodiment, the method comprises, adjusting a ratio of engine braking torque relative to regenerative braking torque applied to the vehicle during a downhill travel based on an estimated grade of the downhill travel and an operator input. In this way, in response to an operator input indicative of a sufficiently long downhill segment, a smaller regenerative braking torque and lower battery charging rate can be used to improve the life and performance of the battery.
In one example, a vehicle operator may indicate an upcoming long downhill travel segment by pressing a button (e.g., a downhill vehicle speed assistance request button). Accordingly, a larger engine braking torque and a smaller regenerative braking torque may be applied to maintain a desired downhill vehicle speed. Additionally, the vehicle battery may be charged at a slower rate over the entire duration of the downhill travel to improve the battery life. In comparison, if no input is received from the operator, a smaller engine braking torque and a larger regenerative braking torque may be used along with a transmission downshift operation to maintain the desired downhill vehicle speed. Additionally, the vehicle battery may be charged at a faster rate to increase the amount of energy captured during downhill travel.
In this way, a larger engine braking torque may be applied to maintain a downhill vehicle speed and a smaller charging rate may be applied to a vehicle battery during a downhill travel with a longer charging opportunity. As a result, battery performance may be improved, thereby improving the useable life of the battery as well as performance and fuel efficiency of a hybrid electric vehicle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.